Fanaelialae
Legend
That explanation is certainly one possibility.The level 20 figures also confirm my suspicions: players will create level 20 builds directly, either to experiment, or play end game. I suspect most level 20 PCs were never actually played all the way through. This seems to confirm that
However, assuming that they tried to filter out characters that were created but never played, level 20 characters would tend to accumulate over time. The other levels are transitory but once you hit 20 that's it. As such, it would make sense for 20th level characters to accumulate over time, despite the dearth of characters in the levels preceding it. Presumably, the only way to remove such a character from the data set at that point would be to delete it.